Organic EL elements have been actively researched and developed. In a fundamental structure of the organic EL element, a layer including a luminescent organic compound (hereinafter also referred to as light-emitting layer) is interposed between a pair of electrodes. The organic EL element has attracted attention as a next-generation flat panel display element owing to characteristics such as feasibility of being thinner and lighter, high speed response to input signals, and capability of direct current low voltage driving. In addition, a display using such a light-emitting element has a feature that it is excellent in contrast and image quality, and has a wide viewing angle. Further, being a planar light source, the organic EL element has been attempted to be applied as a light source such as a backlight of a liquid crystal display and a lighting device.
The emission mechanism of the organic EL element is of a carrier-injection type. That is, by application of voltage with a light-emitting layer interposed between electrodes, electrons and holes injected from the electrodes are recombined to make a light-emitting substance excited, and light is emitted when the excited state relaxes to the ground state. There can be two types of the excited states: a singlet excited state and a triplet excited state. Further, the statistical generation ratio of the singlet excited state to the triplet excited state in a light-emitting element is considered to be 1:3.
In general, the ground state of a light-emitting organic compound is a singlet state. Therefore, light emission from the singlet excited state is referred to as fluorescence because it is caused by electron transition between the same spin multiplicities. On the other hand, light emission from the triplet excited state is referred to as phosphorescence where electron transition occurs between different spin multiplicities. Here, in a compound emitting fluorescence (hereinafter referred to as fluorescent compound), in general, phosphorescence is not observed at room temperature, and only fluorescence is observed. Accordingly, the internal quantum efficiency (the ratio of generated photons to injected carriers) in a light-emitting element including a fluorescent compound is assumed to have a theoretical limit of 25% based on the above ratio of the singlet excited state to the triplet excited state (=1:3).
On the other hand, when a compound emitting phosphorescence (hereinafter referred to as phosphorescent compound) is used, the internal quantum efficiency can be theoretically increased to 100%. That is, higher emission efficiency can be obtained than using a fluorescent compound. For these reasons, a light-emitting element including a phosphorescent compound has been actively developed in recent years in order to achieve a high-efficiency light-emitting element.
As the phosphorescent compound, an organometallic complex that has iridium or the like as a central metal has particularly attracted attention owing to their high phosphorescence quantum efficiency; for example, an organometallic complex that has iridium as a central metal is disclosed as a phosphorescent material in Patent Document 1.
When a light-emitting layer of a light-emitting element is formed using a phosphorescent compound described above, in order to suppress concentration quenching or quenching due to triplet-triplet annihilation in the phosphorescent compound, the light-emitting layer is often formed such that the phosphorescent compound is dispersed in a matrix of another compound. Here, the compound serving as the matrix is called a host, and the compound dispersed in the matrix, such as a phosphorescent compound, is called a guest.
There are generally given several elementary processes for light emission in such a light-emitting element using a phosphorescent compound as a guest, and descriptions of the elementary processes are given below.
(1) The case where an electron and a hole are recombined in a guest molecule, and the guest molecule is excited (direct recombination process).
(1-1) When the excited state of the guest molecule is a triplet excited state, the guest molecule emits phosphorescence.
(1-2) When the excited state of the guest molecule is a singlet excited state, the guest molecule in the singlet excited state undergoes intersystem crossing to a triplet excited state and emits phosphorescence.
In other words, in the direct recombination process in (1), as long as the efficiency of intersystem crossing and the phosphorescence quantum efficiency of the guest molecule are high, high emission efficiency can be obtained.
(2) The case where an electron and a hole are recombined in a host molecule and the host molecule is put in an excited state (energy transfer process).
(2-1) When the excited state of the host molecule is a triplet excited state and the triplet excitation energy level (T1 level) of the host molecule is higher than that of the guest molecule, excitation energy is transferred from the host molecule to the guest molecule, and thus the guest molecule is put in a triplet excited state. The guest molecule in the triplet excited state emits phosphorescence. Note that it is necessary to consider the reverse energy transfer to the triplet excitation energy level (T1 level) of the host molecules. Therefore, the T1 level of the host molecules must be higher than that of the guest molecule.
(2-2) When the excited state of the host molecule is a singlet excited state and the S1 level of the host molecule is higher than the S1 level and T1 level of the guest molecule, excitation energy is transferred from the host molecule to the guest molecule, and thus, the guest molecule is put in a singlet excited state or a triplet excited state. The guest molecule in the triplet excited state emits phosphorescence. In addition, the guest molecule in the singlet excited state undergoes intersystem crossing to a triplet excited state, and emits phosphorescence.
In other words, in the energy transfer process in (2), it is significantly important to efficiently transfer not only the triplet excitation energy but also the singlet excitation energy of the host molecules to the guest molecule.
In view of the above-described energy transfer processes, before the excitation energy of the host molecule is transferred to the guest molecule, when the host molecule itself is deactivated by emitting the excitation energy as light or heat, the emission efficiency is decreased.
<Energy Transfer Process>
Energy transfer processes between molecules are described below in details.
First, as a mechanism of energy transfer between molecules, the following two mechanisms are proposed. A molecule providing excitation energy is referred to as host molecule, while a molecule receiving excitation energy is referred to as guest molecule.
<<Förster Mechanism (Dipole-Dipole Interaction)>>
Förster mechanism (also referred to as Förster resonance energy transfer) does not require direct contact between molecules for energy transfer. Through a resonant phenomenon of dipolar oscillation between a host molecule and a guest molecule, energy transfer occurs. By the resonant phenomenon of dipolar oscillation, the host molecule provides energy to the guest molecule, and thus, the host molecule is put in a ground state and the guest molecule is put in an excited state. The rate constant kh*→g of Förster mechanism is expressed by Formula (1).
                              [                      Formula            ⁢                                                  ⁢                          (              1              )                                ]                ⁢                                                                                                k                                    h              *                        ->            g                          =                                            9000              ⁢                              c                4                            ⁢                              K                2                            ⁢              ϕ              ⁢                                                          ⁢              ln              ⁢                                                          ⁢              10                                      128              ⁢                              π                5                            ⁢                              n                4                            ⁢              N              ⁢                                                          ⁢              τ              ⁢                                                          ⁢                              R                6                                              ⁢                      ∫                                                                                                      f                      h                      ′                                        ⁡                                          (                      v                      )                                                        ⁢                                      ɛ                    g                                    ⁢                                      (                    v                    )                                                                    v                  4                                            ⁢                              ⅆ                v                                                                        (        1        )            
In Formula (1), v denotes a frequency, f′h(v) denotes a normalized emission spectrum of a host molecule (a fluorescent spectrum in energy transfer from a singlet excited state, and a phosphorescent spectrum in energy transfer from a triplet excited state), εg(v) denotes a molar absorption coefficient of a guest molecule, N denotes Avogadro's number, n denotes a refractive index of a medium, R denotes an intermolecular distance between the host molecule and the guest molecule, τ denotes a measured lifetime of an excited state (fluorescence lifetime or phosphorescence lifetime), c denotes the speed of light, φ denotes a luminescence quantum efficiency (a fluorescence quantum efficiency in energy transfer from a singlet excited state, and a phosphorescence quantum efficiency in energy transfer from a triplet excited state) of the host molecule, and K2 denotes a coefficient (0 to 4) of orientation of a transition dipole moment between the host molecule and the guest molecule. Note that K2=⅔ in random orientation.
<<Dexter Mechanism (Electron Exchange Interaction)>>
In Dexter mechanism (also referred to as Dexter electron transfer), a host molecule and a guest molecule are close to a contact effective range where their orbitals overlap, and the host molecule in an excited state and the guest molecule in a ground state exchange their electrons, which leads to energy transfer. The rate constant kh*→g of Dexter mechanism is expressed by Formula (2).
                              [                      Formula            ⁢                                                  ⁢                          (              2              )                                ]                ⁢                                                                                                k                                    h              *                        ->            g                          =                              (                                          2                ⁢                π                            h                        )                    ⁢                      K            2                    ⁢                      exp            ⁡                          (                              -                                                      2                    ⁢                    R                                    L                                            )                                ⁢                      ∫                                                            f                  h                  ′                                ⁡                                  (                  v                  )                                            ⁢                                                ɛ                  g                  ′                                ⁡                                  (                  v                  )                                            ⁢                              ⅆ                v                                                                        (        2        )            
In Formula (2), h denotes a Planck constant, K denotes a constant having an energy dimension, v denotes a frequency, f′h(v) denotes a normalized emission spectrum of a host molecule (a fluorescent spectrum in energy transfer from a singlet excited state, and a phosphorescent spectrum in energy transfer from a triplet excited state), ε′g(v) denotes a normalized absorption spectrum of a guest molecule, L denotes an effective molecular radius, and R denotes an intermolecular distance between the host molecule and the guest molecule.
Here, the efficiency of energy transfer from the host molecule to the guest molecule (energy transfer efficiency ΦET) is thought to be expressed by Formula (3). In the formula, kr denotes a rate constant of a light-emission process of the host molecule (fluorescence in energy transfer from the host molecule in a singlet excited state, and phosphorescence in energy transfer from the host molecule in a triplet excited state), kn denotes a rate constant of a non-light-emission process (thermal deactivation or intersystem crossing), and τ denotes a measured lifetime of the excited state of the host molecule.
                              [                      Formula            ⁢                                                  ⁢                          (              3              )                                ]                ⁢                                                                                                Φ          ET                =                                            k                                                h                  *                                ->                g                                                                    k                r                            +                              k                n                            +                              k                                                      h                    *                                    ->                  g                                                              =                                    k                                                h                  *                                ->                g                                                                    (                                  1                  τ                                )                            +                              k                                                      h                    *                                    ->                  g                                                                                        (        3        )            
First, according to Formula (3), it is found that the energy transfer efficiency ΦET can be increased by further increasing the rate constant kh*→g of energy transfer as compared with another competing rate constant kr+kn(=1/τ). Then, in order to increase the rate constant kh*→g of energy transfer, based on Formulae (1) and (2), in Förster mechanism and Dexter mechanism, it is preferable that an emission spectrum of a host molecule (a fluorescent spectrum in energy transfer from a singlet excited state, and a phosphorescent spectrum in energy transfer from a triplet excited state) largely overlap with an absorption spectrum of a guest molecule (an energy difference between a triplet excited state and a ground state in the usual case of phosphorescence).